How much will Americans do to help clean up the air, reduce global warming and promote energy independence? One test will come in November, when voters in California, the nation's biggest state, decide on a ballot measure that would raise $4 billion for alternative energy investments by taxing oil drilling. The explosive battle over Proposition 87, known as the Clean Alternative Energy Act, has turned into the costliest initiative campaign in American history  with $105 million spent so far, mostly on television spots.

Some call it Bing vs. Big Oil. The bulk of the money behind the measure comes from Stephen Bing, 41, a Hollywood producer who was, until now, best known for fathering actress Elizabeth Hurley's baby out of wedlock. Bing, heir to a $600 million New York real estate fortune and a generous contributor to Democratic candidates, has spent $40 million of his own money on Prop 87 so far. Though he won't talk to the media, he has recruited two prominent spokesmen, former President Bill Clinton and former Vice President Al Gore, as well as such Hollywood supporters as Julia Roberts, Geena Davis and Jamie Lee Curtis.

Both Clinton and Gore appear in TV ads for the initiative. "Prop 87 is the one thing Californians can do now to clean up the air, help stop the climate crisis and free us from foreign oil," Gore says in his spot. Clinton, accompanied by Davis (who starred as the first female President on last season's TV series Commander in Chief) appeared at a Prop 87 rally at the University of California in Los Angeles last week. "California is being given an opportunity and an obligation to do something remarkable to save the planet," Clinton told the crowd of 5,000. "You are dangerously dependent on unstable sources of oil, and your air is too polluted." Silicon Valley bigwigs, including Google founder Larry Page and venture capitalists John Doerr and Vinod Khosla  wagering that clean tech will be the next bonanza  have also ponied up several million in favor of the initiative.

If the measure passes, oil drilled in California, the world's sixth largest economy and the fourth largest oil-producing state in the U.S., would be taxed at a rate from 1.5% to 6%, depending on global crude prices. The proceeds, capped at $4 billion, would fund a state agency to sponsor research and projects in wind, solar, ethanol and other energy alternatives. The idea of putting a tax on oil extraction is not new  both Texas and Alaska have one  but California's idea to use the money for alternative energy projects is.